1. Field of the Invention
The present invention relates to a partial discharge detection device that detects partial discharge which is generated in a gas-insulated equipment, and more particularly to a partial discharge detection device which is capable of detecting, with high sensitivity, electromagnetic waves caused by the partial discharge generation.
2. Description of the Related Art
Generally, gas-insulated equipments such as gas-insulated switchgears, gas-insulated generating lines, and gas-insulated transformers are employed in transformer substations. Gas-insulated equipments are devices in which a high voltage conductor is housed in a hermetically sealed metal container which is charged with an insulating gas and supported by means of an insulator. It is known that, in a gas-insulated equipment, when there is a defective part with which there is poor contact in the metal container or which is invaded by a metallic foreign body, partial discharge from the defective part is generated.
When partial discharge in a gas-insulated equipment is neglected, before long insulation damage is incurred and there is the danger that the damage will develop into a serious accident. Hence, it is essential to discover partial discharge at an early stage and take precautions against serious accidents by implementing countermeasures of some kind such as the repair of defective parts. Therefore, a partial discharge detection device that detects partial discharge in a gas-insulated equipment has been proposed as a technology for preventive maintenance for insulation diagnosis of the gas-insulated equipment.
Methods for detecting partial discharge in a gas-insulated equipment include methods for detecting electric current, electromagnetic waves, sound, vibration, and light, and so forth. Among such detection devices, devices for determining the existence of partial discharge by detecting electromagnetic wave signals caused by partial discharge have favorable detection sensitivity, a good S/N ratio, and a wide detection range and so forth and have garnered attention. In particular, because the electromagnetic waves caused by partial discharge include signals extending from several dozen MHz to several GHz, procedures for detecting UHF-band electromagnetic waves (300 MHz to 3 GHz) have come into the mainstream.
A conventional example of a partial discharge detection device will be described here using FIG. 5. As shown in FIG. 5, in a gas-insulated equipment 10, an insulating gas such as SF6 gas is enclosed in a ground-potential hermetically sealed metal container 22 which houses a high-voltage conductor 25, and the high-voltage conductor 25 is supported by an insulating spacer 24 comprising an insulator. The partial discharge detection device which is inserted in the gas-insulated equipment 10 is constituted by an internal detector 27, a matching circuit 28, an amplifier 29, a measurement tool 30, and a determination tool 31.
Among these components, the internal detector 27 is an electrode that detects electromagnetic waves 36 in the metal container 22 and is installed on a flange 26 of a low electric field hand hole of the metal container 22. Furthermore, the matching circuit 28 comprises a filter or the like and is adapted to pull the detection signal of the internal detector 27 outside the metal container 22 via a pullout section and to specify the frequency band of the detection signal. The amplifier 29, measurement tool 30, and determination tool 31 are sequentially connected to the matching circuit 28. The amplifier 29 performs processing to amplify the detection signal whose frequency band has been specified by the matching circuit 28 and the processed signal is measured by the measurement tool 30 so the existence of partial discharge which is generated inside the gas-insulated equipment 10 is ultimately determined by the determination tool 31.
The partial discharge detection device as described above operates as follows. First, when partial discharge is generated in a defective part 35 inside the gas-insulated equipment 10, electromagnetic waves 36 from several dozen MHz to several dozen GHz due to partial discharge in the metal container 22 are generated. Here, the hermetically sealed metal container 22 propagates electromagnetic waves 36 as a result of waveguide theory. Further, the internal detector 27 detects electromagnetic waves 36 in the metal container 22 and the matching circuit 28 specifies the frequency band of the electromagnetic waves 36 thus detected. In addition, the amplifier 29 and measurement tool 30 perform signal processing on the specified frequency and the determination tool 31 judges the existence of an anomaly with the gas-insulated equipment.
Thus, by detecting electromagnetic waves 36 by means of the internal detector 27, the partial discharge detection device is able to detect, with high sensitivity, partial discharge by defective part 35 which is generated in the gas-insulated equipment 10. Moreover, in the conventional example in FIG. 5, because the internal detector 27 is installed inside the metal container 22, a noise signal from outside the metal container 22 can be attenuated and the partial discharge in the metal container 22 can be detected with a favorable S/N ratio.
Further, the electromagnetic waves which progress within the waveguide must satisfy boundary conditions at the boundary plane of the waveguide and Maxwell's electromagnetic equations. Hence, when electromagnetic waves are propagating within the metal container 22 which is the waveguide, the boundary conditions of the metal surface, that is, the conditions that the electric field be perpendicular to the metal surface and that the magnetic field be parallel must be satisfied. The following progressive waves exist as electromagnetic waves which satisfy these conditions.
First, in the case of electromagnetic waves for which the direction of travel is the axial direction of the metal container 22, progressive waves which do not possess an electric field or magnetic field component in the direction of travel are called ‘Transverse Electromagnetic Waves (TEM waves). However, as a result of the boundary conditions, although the electric field travel direction component is zero, there exist also Transverse Electric waves (TE waves) in which a magnetic field travel direction component exists or Transverse Magnetic waves (TM waves) in which the magnetic field travel direction component is zero but in which an electric field travel direction component exists. Various modes exist for such TE waves and TM waves.
In a rectangular waveguide, a cut-off frequency which is determined by the shape of the waveguide exists and electromagnetic waves of a lower frequency than TE10 mode which is the lowest frequency are attenuated. Hence, the electromagnetic waves of a frequency equal to or less than the cut-off frequency are attenuated greatly and not propagated. For example, in cases where a rectangular waveguide and a coaxial waveguide are connected by means of a microwave circuit or the like, for example, it is said that only electromagnetic waves at or below the cut-off wavelength which can be propagated by the waveguide can be converted to electromagnetic waves which propagate along the coaxial waveguide and that electromagnetic waves at or above the cut-off wavelength cannot propagate along the coaxial waveguide.
With regard to this point, the coaxial waveguide converter that appears in ‘Basis for Microwave Engineering’, Hiratani et al., Japan Science and Technology Publishing) will be described as a specific example (See FIGS. 6 and 7). In FIG. 6, 40 is a rectangular waveguide made of metal at one end of which a metal short-circuit plate 41 is provided and at the other end of which a waveguide connection flange 42 for a connection with another waveguide is provided.
Furthermore, a coaxial cable connector 43 is provided on one face of the waveguide 40 (upper side in FIG. 6). FIG. 7 shows a cross-sectional view of the coaxial waveguide converter and the impedance which corresponds with the coaxial conversion of the rectangular waveguide differs from the coaxial characteristic impedance. Therefore, in order to alleviate the mismatch caused by the disparity between the two impedances, L and d shown in FIG. 7 are adjusted through experimentation. Numeral 44 denotes electrical wiring.
The operation of the coaxial waveguide converter will be described next. The electromagnetic waves which are propagated by the waveguide (not shown) which is connected to the waveguide connection flange 42 are converted into electromagnetic waves which propagate along a coaxial structure line constituted by a coaxial inner conductor 43a and a coaxial outer conductor 43b. Here, the coaxial waveguide converter in FIG. 6 propagates only electromagnetic waves at or below the cut-off wavelength which can be propagated within the waveguide to a coaxial line but is unable to propagate electromagnetic waves at or above the cut-off wavelength to the coaxial line.
In addition, as a conventional technology for a partial discharge detection device which detects the existence of partial discharge from electromagnetic waves which are produced by a gas-insulated equipment, a technology that detects electromagnetic waves which leak in from the opening in an impedance discontinuous plane of a spacer or bushing by means of an antenna which is placed close to the opening has been proposed. Otherwise, a method that involves disposing a slit antenna or dipole antenna along a spacer flange as per the technology of Japanese Application Laid Open No. H3-78429 is also known. According to these methods, there is no need to attach a detector inside the metal container of the gas-insulated equipment and there is therefore the advantage that electromagnetic waves can be detected using a simple constitution.
As mentioned hereinabove, in a gas-insulated equipment 10 that comprises a hermetically sealed metal container 22, because electromagnetic waves are propagated within the metal container 22 on the basis of the waveguide theory, the electromagnetic waves can be detected with high sensitivity by the internal detector 27. However, the internal detector 27 shown in FIG. 5 is provided inside the metal container 22 of the gas-insulated equipment 10. Hence, it has proven difficult to retro-fit an internal detector 27 to a gas-insulated equipment which does not have the internal detector 27 attached beforehand and hard to detect electromagnetic waves.
Therefore, as a method that makes it possible to detect electromagnetic waves with a simple constitution, a test that detects electromagnetic waves which have leaked from an opening in an impedance discontinuous plane such as a spacer or bushing is carried out and a method in which an antenna is installed in an external space of the gas-insulated equipment and a method that involves disposing a slit antenna or dipole antenna along the outer circumference of the spacer have been proposed.
However, the following problems with these methods were noted. That is, with a slit antenna, an electrical connection to the metal container was essential. Further, because the impedance is discontinuous, the detection sensitivity of both methods was low and there was a high susceptibility to the effects of external noise. Hence, the S/N ratio was inadequate and the partial discharge could not be detected highly accurately.
Moreover, in the case of a rectangular waveguide, the cut-off frequency is decided by the shape of the waveguide and the wavelength of the electromagnetic waves propagated by the long side dimension of the waveguide is constrained, meaning that propagation of electromagnetic waves of wavelengths equal to or more than two times the long side dimension of the waveguide cross-section is inefficient. More specifically, in order to propagate electromagnetic wave signals in the UHF band (300 MHz to 3 GHz, wavelength: 100 mm to 1000 mm), a rectangular waveguide with a long side that is equal to or more than 500 mm is required. Therefore, the coaxial waveguide converter shown in FIGS. 6 and 7 has a large form and, when the form is integrated in a partial discharge detection device, this has the inconvenience of enlarging the device.